The invention relates to gamma ray detector and methods of gamma ray detection, and in particular to scintillator-based gamma ray detectors and methods of gamma ray detection for use in gamma ray spectrometers.
Gamma ray spectrometers are used in a wide variety of applications, for example to identify and monitor gamma ray sources in scientific, industrial, and environmental monitoring applications, e.g. for security screening of personnel and cargo at border crossings, or to search generally for orphaned radioactive sources. A common class of gamma ray spectrometers is based on organic (plastic) or inorganic (crystal) scintillator materials.
FIG. 1 shows an example of a conventional scintillation spectrometer 2. The spectrometer comprises a scintillation body 4 which scintillates when a gamma ray is absorbed within it. Some common scintillator materials include crystals, such as thallium doped sodium iodide (NaI(Tl)), and plastics, such as polyvinyltoluene (PVT).
The scintillation body 4 is surrounded by a diffuse reflector 6. The diffuse reflector 6 may comprise a white powder, such as Al2O3, packed around the scintillation body 4, or a white tape, such as Teflon tape, wrapped around the scintillator body 4. For large plastic scintillation bodies it is known to wrap the scintillation body in crinkled metal foil and it is also known to optically bond a reflector to edges of a plastic scintillation body that have poor optical characteristics.
Gamma rays from a source enter the spectrometer and interact with the scintillation body 4 in scintillation events in which lower-energy photons are generated, e.g. optical photons. The intensity of the flash of lower-energy photons depends on the amount of the energy of the incident gamma ray deposited in the body. The scintillation body 4 is optically coupled to a photomultiplier tube (PMT) 10 on an end opposite a front face 8 of the scintillation body 4. The PMT 10 is for detecting photons generated in the scintillation body 4 in gamma ray detection events. Thus the PMT 10 is operable to output a signal S indicative of the intensity of the scintillation flash generated in the body 4 in response to each gamma ray interaction.
Output signals S from the PMT 10 are routed to a spectrum analyser 11, e.g. a multi-channel analyser. The amplitudes of the respective output signals S are indicative of the energy of the corresponding incident gamma rays deposited in the scintillation body. The relationship between an energy deposit in the scintillation body 4 and a resulting output signal S is defined by a response function of the detector.
The spectrum analyser 11 is operable to process the output signals S received from the PMT in a given integration time (or in an accumulating manner) and to generate an energy loss spectrum for the corresponding detection events. To do this the spectrum analyser 11 converts the measured output signals S to estimates of the energy deposited D in the gamma ray detector in the corresponding events.
Some applications of gamma ray spectrometers require large-volume detectors. Plastic scintillation material (e.g. PVT) is widely used to provide large-volume detectors for both charged particle and gamma ray detection. Other plastic based scintillators include materials that can be loaded with either high-Z fluor materials such as those described by Cherepy et al in Nuclear Science Symposium, Paper N41-3 Conference record (Anaheim) 2012 [4], including 9-vinyl carbazole, or other high-Z loaded nano-composite materials. For security applications, detectors having volumes of up to ˜30 liters or so are in widespread use to detect gamma ray emitting isotopes which might be concealed in cargo at ports and other border-crossing points. These generally take the form of large rectangular slabs of material and are frequently viewed by two or more small photomultipliers in order to compensate for the intrinsically poor light-collection efficiency of the design. The detectors can rely on coincident signal detection from the two photomultiplier tubes to distinguish between weak optical signals associated with gamma ray detection events and photo-multiplier noise. Following the detection of a potential radio-active material, it is typically necessary to detain the relevant cargo/vehicle for secondary inspection, for example using a hand-held spectrometer of sufficient quality to be able to identify the nature of the radioactive material reliably.
Previously, plastic scintillator materials have not been regarded as suitable for applications where spectroscopic information is required. However, through the application of careful optical design and techniques for de-convolving raw energy-loss spectra and the use of a continuous stabilization and calibration techniques (e.g. as disclosed in GB 2 437 979 [1], GB 2 418 015 [2] and GB 2 463 707 [3]), it has become possible to extract useful spectroscopic information from large volume plastic detectors.
Nonetheless, there remains a desire to provide gamma ray detectors from which still further improved spectroscopic information can be obtained.